People, vehicles, aircraft and ships may be exposed to projectiles launched from barreled weapons such as guns. Potential targets may also be exposed to projectiles created by munitions that utilize explosive detonations to form metal slugs. They may be struck by fragments generated by munitions such as artillery shells filled with explosives. Although not present on terrestrial battlefields, spacecraft are at continuous risk of impacts by projectiles such as dust and larger debris traveling at extremely high velocities. Regardless of their velocity, protection against projectiles is typically provided by armor.
Armor Including Multiple Layers
The current art for protection against supersonic projectiles usually involves armor consisting of at least two components. This is because homogeneous materials such as steel and ceramics typically require greater weight and thickness to stop a projectile than is required by armor assemblies utilizing two or more layers of different materials.
Considerable research and combat experience have consistently shown that the most efficient armors are made by joining a hard, dense impact surface or “strike face” to at least one backing layer that supports the first layer. To stop conventional projectile threats as exemplified by bullets and bomb fragments, ceramic strike faces are commonly used, but hard metal alloy strike faces also are effective in resisting many projectiles.
Hard strike faces increase likelihood that projectiles will shatter due to stress waves reflecting back and forth within them caused by impact. The fragments have much lower kinetic energy and momentum than the intact projectile. Early projectile disintegration enables load created by their impact to spread over a larger area within the armor, thereby reducing local stresses.
Hard, dense strike faces also serve to resist penetration for a longer time, thus allowing more momentum to be transferred from the projectile to the armor assembly. Longer residence or “dwell” time also allows local stresses generated by projectile impact to spread over a wider area. The result is similar to what occurs when incident projectiles shatter.
A wide range of materials and material combinations are employed as secondary or backing layers. Backing layers that resist bending are essential when ceramic strike faces are used because of the low tolerance for deformation inherent to ceramics.
Metal alloy and fiber-reinforced matrix composite materials are commonly selected as backing layers for both ceramic and metal strike faces. Composite materials typically offer higher strength to weight ratios than metal alloys, so composite backing layers are generally favored in applications where minimum weight is essential. Matrix materials may be organic resins such as epoxy and phenolic compounds, or alternatively may feature metals. Fiber materials range from graphite whiskers to organic fibers such as polyamides and modified thermoplastic resins such as polypropylene.
Despite the general effectiveness of armors made with the present art, numerous problems and shortcomings nonetheless remain. Projectiles with high kinetic energy often require unacceptably high weights of protective armor made with the current art to prevent penetration into people and aircraft, as well as into most vehicle types. High costs associated with ceramics and metal-matrix materials discourage their use in many applications. Other shortcomings become apparent when specific applications are examined.
If projectiles are sufficiently energetic, stress waves propagating in target materials will generate shear and compressive fractures. If penetration is resisted, stress waves may still produce severe deformation of the rear surface, resulting in a bulge, or so weakening the impact area that the armor will fail if another projectile strikes it.
If cracks propagate through the target, the rear surface (the surface opposite the impact surface) may detach or spall even if the projectile itself does not penetrate completely. Either spalling or complete penetration through shear failure in target materials is typical of dense projectiles having high length to diameter ratios (generally referred to as “long rod penetrators”).
Stress waves propagating through armor are transmitted into people or other objects if they are in contact. Bulging of the armor's rear surface can perforate body tissue, as can spall generated by projectile impact on the strike face. All of these localized armor failures can inflict severe or lethal injury to people and serious damage to equipment, even when incident projectiles fail to penetrate.
Sniper bullets fired by ordinary rifles typically use bullets with cores consisting of tungsten carbide or steels with high degrees of hardness. Protecting only the chest and torso of a soldier against such bullets requires armor using the current art significantly exceeding five kilograms. Helmets with this level of protection would be too heavy for necks and shoulders to support if made using the current art.
The large vulnerable area of aircraft and vehicles, combined with the weight of armor made with the current art that is required to stop projectiles typically threatening these targets, force designers to limit armor usage in order to leave weight and space available for fuel and payloads. Projectiles typically fired at aircraft, vehicles and naval vessels are larger than those fired from rifles. Larger projectiles often have explosive fillings and other features that enhance penetrating capability beyond what is possible with small arms. The difficulty of resisting penetration is magnified when both mass and velocity of threat projectiles increases.
The unanswered challenges posed by current projectile threats are retarding the development of new vehicles and aircraft as designers struggle to meet performance and mission requirements while providing adequate protection. For the foregoing reasons, many potential users would welcome new materials and armor assemblies that would prevent projectile penetration with significantly less weight and with thickness no greater than required with armors of the present art.
Through recent material and design innovations, means for achieving the desired objectives are now available. These means involve proper selection and arrangement of suitable materials in novel ways. Innovative means of mounting the armor assembly variants that are part of the invention can further enhance projectile penetration resistance and well as mitigate shock associated with projectile impact.
Strike Face Materials
Ceramic strike faces typically possess higher hardness levels than are achievable with most metals. Hardness and material strength are important to resisting penetration by relatively small projectiles traveling at velocities on the order of 1 kilometer per second (km/s). Against pointed projectiles, ceramic strike faces generally extend the time of contact prior to penetration longer than occurs with metal strike faces.
Ceramics available currently are generally less dense than steel and at least as hard. As noted heretofore, ceramics also offer characteristically high acoustic speeds. This is important for rapidly dissipating projectile energy and localized contact stresses under the point of impact. The acoustic speed of alumina, for example, is approximately 10 kilometers per second. This is roughly twice the acoustic speed of steel, and 70% higher than for aluminum.
Ceramic strike face materials have drawbacks that affect performance and usage, however. Generally, ceramics are quite expensive. This is particularly the case for ceramics designated as “armor grade” silicon carbide, titanium diboride, tungsten carbide and alumina. These ceramics require careful process control during manufacturing processes that occur at high temperatures, and thus are prone to inconsistent properties between one batch and another of the same nominal composition.
Low bending tolerance inherent to all ceramics currently used in armor was noted previously. Another significant risk is that cracks may form in ceramics that have been dropped without the user knowing that penetration resistance has been degraded. This vulnerability is particular significant when the threat of multiple projectile impacts on ceramic armor is present.
Ceramics typically cannot withstand repeated projectile impacts within 2 centimeters of a previous hit. Ceramics are also less effective than metal when struck by blunt or flat projectiles. Furthermore, ceramics are particularly sensitive to impacts by explosively formed penetrators, or “EFPs”.
Among commonly used ceramic armor materials, all but tungsten carbide armors tend to shatter or disintegrate into small particles (comminute) at high projectile impact velocities. Shattering negates the advantage of high acoustic speed because stress waves reflect at the new boundaries created by the cracks. Stress waves emanating from the projectile impact zone thus cannot dissipate into surrounding ceramic material.
Despite the heavier weight required, metals offer some advantages over ceramic strike faces. Most metals resist shattering under projectile impact. Certain steels can be processed in ways that provide them with high degrees of hardness and high tensile strengths while being somewhat more resistant to bending stresses. Iron alloys with significant additions of chromium and molybdenum also display both hardness and tolerance for localized bending. Mechanical properties of metal armor components are typically not degraded by subsonic collision impact. Structures are generally able to support the extra weight involved with use of metal strike faces.
Although expensive, tungsten offers particular advantages. It is strong but is more dense than steel. High density is important because of a quality called impedance. Impedance is defined as the mathematical product of density and velocity of the shock wave as it travels through the material. When the impedance of the target is higher than that of the projectile, the contact load at the impact surface is substantially reduced. This happens because shock waves generated within both the target and the projectile reflect from their surfaces, which sends negative pressure or relaxation waves back to the impact surface within the material possessing higher impedance.
As is the case with ceramics, hard metal strike faces also have their shortcomings. In addition to unfavorable comparisons with respect to weight and hardness, performance advantages of tungsten and tungsten alloys are often offset by costs and limitations in supply. Supply of other dense metals with high strength such as tantalum is far too small to consider in armor. As also noted previously, only tungsten has an acoustic speed nearly as high as that of most ceramic materials.